This invention relates to transparent coatings. More specifically this application relates to optically clear abrasion (or scratch) and chemical-resistant films or coatings for use on polymer surfaces. These coatings can also be used on metallic substrates for improving their hardness and corrosion (or barrier) properties.
The present invention fulfills the need for scratch resistant coatings on plastic substrates, which are needed in a variety of applications such as, high index ophthalmic and sportswear lenses, automobile side windows and transparencies for aircraft cockpits. Plastic substrates such as, polycarbonate and acrylic, can scratch easily and lose transparency quickly during daily use and maintenance. Hard and optically transparent coatings for plastic substrates possess significant market potential. However, a satisfactory coating technology is not yet available.
The object of this invention is to develop a easily applied coating technology, which can satisfy the need of ophthalmic, automobile and aircraft industries. The existing coatings on plastic substrates have limitations such as low refractive index and short operational life. Incorporation of a high volume fraction of large ceramic particles in a polymer nanocomposite matrix can result in ceramic-like coatings, which will have higher hardness and better durability than those of polymer nanocomposite coatings. However, large ceramic particles embedded in a polymer matrix will scatter light because of the difference in their refractive indices and the inhomogeneous interface between the particle and the matrix. In this invention, smooth, dense and transparent films (with no haze) were fabricated by chemically modifying the surface of ceramic particles and by engineering the refractive index of polymer nanocomposite matrices.
The overall object of this invention is to provide high abrasion and chemically resistant solution-based coatings for polymeric substrates, enabling them to become attractive alternatives to inorganic glasses in applications such as, automobile side windows and head lamps, high index ophthalmic and sportswear lenses, and transparencies for aircraft cockpits and cabins. This is because even though glass is hard and optically transparent, it has disadvantages in weight and safety. Many optically transparent polymers, such as polycarbonate, polyetherimide and acrylic, have lower densities and a much higher toughness than glass. It has been estimated that the use of polycarbonate substrates over glass in automobile window applications will reduce the total weight by 40%. Additionally, polycarbonate materials have high impact resistance and provide a high degree of design freedom. The single biggest roadblock to the widespread use of polycarbonates is the lack of an adequately hard and optically transparent coating that can provide scratch as well as wear resistance. Without protection, the material can suffer from severe surface damage and lose transparency quickly during daily use and maintenance.
Several organic (e.g., urethane) and silicone-based abrasion resistant coatings have been developed for plastics used in ophthalmic and non-ophthalmic lens applications. However, these coatings do not meet the stringent requirements for both automobile side windows and aircraft transparency applications. Additionally, the eye lens industry is focusing on producing high index lenses (refractive index, nD˜1.6-1.7), which require high refractive index (nD=1.63-1.68) wear resistant coatings. The refractive index of presently available organic abrasion resistant coatings is ˜1.5, making them unsuitable for high index lenses. Therefore, there is an immediate need for coatings that have high refractive index as well as excellent abrasion resistance.
Recently, optical coatings on polymeric substrates have been developed using two approaches: wet chemical methods (e.g., sol-gel process), and vacuum/gas phase processes. The latter approach is based on the deposition of an inorganic material by a plasma torch. Solution processes have been used to develop polymer-nanophase oxide composite materials by either dispersing surface modified nanoparticulates (diameter ˜10-20 nm) in siloxane polymers, or sol-gel based organic/inorganic hybrid networks called ‘Ceramers’.
Coatings on polymers by incorporating oxide or hydroxide nanoparticles in epoxysilane have been developed (See e.g.: M. Mennig, P. W. Oliveira, A. Frantzen and H. Schmidt, Thin Solid Films, 351, 225 (1999)). Methacrylate functionalized silanes and nanoscaled boehmite particles were used for the preparation of UV curable hard coatings by the sol-gel technique. An inorganic network was formed as a result of controlled hydrolysis and condensation of the methacryloxysilanes in the presence of nanoparticles (particle size ˜15 nm). Transparent coatings were prepared on plastic substrates (e.g., polycarbonate, PMMA). The coatings showed excellent adhesion and good abrasion resistance. Loss in transmission was 10% after 1000 cycles in a Taber abraser test (CS 10F rolls, 5.4 N). Others have synthesized polymer nanocomposite coatings by mixing electrostatically stabilized oxide particulate sots with 3-glycidoxypropyltrimethoxysilane (GPTS). An amino functionalized alkoxysilane was used as condensation catalyst and the nanocomposite material was thermally cured at 130° C. after spin coating on pretreated polycarbonate substrates. After 1000 cycles Taber abrasion test (CS-10F, 500 g), loss in transmittance due to scattering was 2-6%. However, the index of refraction of these coatings was too low for many optical applications.
Others have synthesized a variety of sol-gel coatings (see e.g., U.S. Pat. No. 5,316,855). The best abrasion resistance was obtained for sol-gel coatings synthesized using diethylenetriamine (DETA) that was functionalized at its amine groups with 3-isocyanatopropyltriethoxysilane. The functionalized DETA (F-DETA) was mixed with tetramethylorthosilicate (TMOS). These reactants undergo a comparable sol-gel reaction similar to typical metal alkoxide chemistry, thereby producing an insoluble network material containing a good dispersion of the functionalized organic, along with the hydrolyzed and condensed silicon alkoxide group. The final coating on a polycarbonate substrate was transparent and few microns in thickness. The transmission of these coatings was 98.5 after a 500-cycle Taber test using CS 10 wheels, 500 g load per wheel. However, the refractive index of such coatings was ˜1.5, and the shelf life of the coating formulations was only a few days. It should be noted that such a short life of the formulation is unacceptable for commercial applications. Additionally, automobile manufacturers require the life of coatings to be at least 8 years. It is generally accepted that pure sol-gel coatings may not be able to sustain the tough standards required for practical applications.
To utilize the full potential of polymer nanocomposite coatings for optical applications, including high index ophthalmic lenses, relatively large-sized (at least 0.1-0.4 microns) ceramic wear resistant particles (such as alumina) need to be incorporated in a polymer nanocomposite matrix. The wear properties of polymer nanocomposite coatings will be dramatically enhanced because of the larger wear surface (4πr2) of large particles in comparison to that of nanoparticles. Large particles offer more resistance to deformation, while nanoparticles simply get displaced and provide little resistance to shear forces. Additionally, incorporating ceramic particles will also enhance the coating life; simultaneously, coatings consisting of high volume fraction of alumina particles will have a higher refractive index than that of siloxane-based coatings, if particles are dispersed homogeneously.
However, a majority of ceramic particles cannot be used in existing siloxane or polymer-based coatings because they will scatter light. Ceramic particles will scatter according to the following equation:Scatter intensity of light=I∝(ΔnD)2V2where, V is the volume of the scattering particle, and ΔnD is the difference in the refractive index of the particle and the medium of suspension.
Scattering leads to severe loss of optical transparency; as a result, a siloxane coating which has an nD of 1.5 containing just a few large-sized ceramic particles dispersed in it, will be opaque. On the other hand, the scattered light intensity of large-sized particles will be insignificant if ΔnD is negligible, which will be the case for a matrix that has the same value as of ceramic particles (nD of Al2O3 is ˜1.7). This is also the refractive index of coatings needed for high-index ophthalmic lenses.
In the development of the present invention we have focused our efforts on obtaining optically transparent nanocomposite films containing alumina particles by engineering the surface of particles, and by manipulating the refractive index of the nanocomposite matrix. The present development includes:                a) Formation of coatings with a refractive index in the range of 1.66-1.68.        b) Formation of optically transparent coatings consisting of alumina particles in a nanocomposite matrix and        c) Demonstration of improved mechanical properties in coatings by the addition of particles compared to coatings fabricated without incorporating alumina particles.        
The present invention thus provides the synthesis of optically clear nanocomposite films consisting of large-sized alumina particles/aggregates (average size>0.1 μm).